Optical fibers are rapidly becoming the medium of choice for long distance transmission of information.
An important parameter of optical fibers for the transmission of optical pulses is the chromatic dispersion as a function of distance, hereafter referred to as the dispersion map. For the design of high capacity fiber optic transmission systems as well as for the upgrade of existing installed systems, it has become essential to be able to measure dispersion maps in a fast, accurate and convenient manner.
There have been attempts to measure the dispersion based on modulational instability or phase-matching of four-wave-mixing (FWM) products, either in transmission or reflection. All these approaches require extensive, time consuming data collection, and the spatial information has to be extracted from a convoluted, and potentially ambiguous, amplitude profile. The optical-time-domain-reflection (OTDR)-like technique, described in Opt. Lett. 21, 1724 (1996) by L. F. Mollenauer, P. V. Mamysher, and M. J. Neubelt, on the other hand, completely and uniquely overcomes those disadvantages. As previously demonstrated, access to only one end of the fiber is required, the data acquisition takes place in a couple of seconds, the spatial information is direct, unambiguous, and of high resolution, and the dispersion uncertainty is small. This technique has already been used for quality control in the manufacture of dispersion-shifted (DS) and dispersion compensating (DC) fiber, and for the evaluation of installed fibers in the field.
The OTDR-like technique can be sketched as follows: A relatively strong sub-microsecond pulse consisting of two frequencies, .omega..sub.1 and .omega..sub.2, that are copolarized, is sent into the fiber under test to produce FWM signals, either at the Stokes frequency .omega..sub.S =2.omega..sub.1 -.omega..sub.2 or the anti-Stokes frequency .omega..sub.A =2.omega..sub.2 -.omega..sub.1. The phase-mismatch .delta.k due to chromatic dispersion causes spatial oscillations of the fields at .omega..sub.S and .omega..sub.A, e.g. .delta.k(.lambda..sub.S, z)=-2.pi.cD(.lambda..sub.1, z)(.delta..omega./.omega..sub.1).sup.2, where z is the distance into the fiber, c is the speed of light, D is the dispersion parameter, and .delta..omega.=.omega..sub.2 -.omega..sub.1. Once again, it is important to note that this relation is exactly independent of the third order dispersion. As observed in Rayleigh backscattering, the spatial oscillations of the sidebands become temporal oscillations, whose frequency at time t is proportional to D(.lambda..sub.1, z=ct/2n).